MEMS devices, i.e., miniature devices the size of an integrated circuit, have been introduced into a wide variety of consumer and industrial products that require small devices. One such use of a MEMS device is an accelerometer used in automobiles to detect a car accident. Upon such detection, an air bag may be deployed. The MEMs device has two main portions: 1) a stationary portion; and 2) a movable portion suspended by a spring (i.e., a flexible material) that is coupled to the stationary portion by an anchor. The stationary portion and the movable portion have each have fingers (protrusions) which are interdigitated among each other. In other words, each finger of the stationary portion is surrounded by a finger of the movable portion and separated by a gap. When the car decelerates, for example, from 60 miles per hour (mph) to 0 mph, the gaps between the fingers changes. The change in the gap width is detected by measuring the capacitance between the gaps. Upon a change in capacitance or capacitance threshold being met an action, such as airbag deployment, occurs.
Typically, the gaps are approximately 1-2 microns in width. If any particles are introduced into the gaps the accelerometer will not function properly. One solution to keep particles out of the gaps of the accelerometer is to put a cap wafer on top of the accelerometer. This can be performed by gluing a cap wafer using a glass frit layer over the gaps. One problem with this approach is that it results in a large die size for the accelerometer.
Another approach is to form a layer to seal the gap using plasma enhanced chemical vapor deposition (PECVD) or low pressure chemical vapor deposition (LPCVD). However, both PECVD and LPCVD are performed in vacuum environments and will result in the gap being at vacuum. When the gap's pressure is at vacuum the accelerometer is underdamped and oscillates. This is undesirable because it decreases the performance of the accelerometer. Therefore, a need exists for preventing particles from entering the gap while not underdamping the oscillator.
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